The present invention relates to a solventless underfill material and an electronic component as an application of the underfill material. More particularly, it relates to an underfill material for chip-on-flex or chip-on-film (COF) mounting which comprises a low-elasticity organic material and is used to fill the gap between a flexible printed circuit board (FPC) composed of a polyimide film substrate and a thin copper film circuit formed thereon and an IC chip mounted thereon and to an electronic component having the underfill material.
As the level of integration of semiconductor chips increases, flip chip packages in which a chip and a substrate are connected via bumps have been taking the place of chip-on-board packages such as pin grid array packages in which a chip and a substrate are connected by wire bonding. Spot sealing of the flip chip packages is carried out by using, as an underfill sealant, a flowable solventless one-pack type epoxy resin composition.
With the increasing integration scale of circuits, materials of the substrate have been changing from rigid materials such as glass-epoxy laminates to flexible films, typically polyimide films. However, conventionally employed underfill materials have insufficient adhesion to a polyimide film due to their high tensile modulus and therefore lack sufficient reliability.
The increasing degree of integration with the ever evolving functions of portable equipment is generating demands for both a copper layer and a polyimide film making FPCs to have a reduced thickness and for the underfill material to have a further decreased elastic modulus.
JP-A-9-153570 proposes an underfill material containing butadiene rubber particles and thereby having a reduced elastic modulus, but the butadiene rubber particles are difficult to disperse uniformly only to provide an underfill material with insufficient reliability in long term use.
An object of the present invention is to provide a solventless underfill material which exhibits sufficient adhesion to both a silicon wafer and a polyimide film and satisfactory molding properties, such as a low viscosity and capability of penetrating in a relatively short time, and is applicable to an FPC composed of a thin copper layer and a polyimide film.
Another object of the present invention is to provide an electronic component which uses an underfill material of low elastic modulus.
The present invention relates to a solventless non-filler underfill material for COF mounting comprising an organic material, which is used to fill the gap between an FPC having a polyimide film substrate and a copper circuit layer having a thickness of 9 xcexcm or smaller and an IC chip mounted on the FPC, exhibits such adhesion as to destroy a silicon wafer in a polyimide film/silicon wafer adhesion test, and provides a cured product having a tensile modulus of 150 kg/mm2 or less.
The present invention also relates to an electronic component having an IC chip fixed with the underfill material for COF mounting.
In this specification, xe2x80x9chalf-esterificationxe2x80x9d means esterification of tetracarboxylic dianhydride to dicarboxylic acid diester. xe2x80x9cPenetrabilityxe2x80x9d, xe2x80x9cpenetrationxe2x80x9d and xe2x80x9cpenetratingxe2x80x9d mean that the epoxy resin composition flows inwardly to fill a gap between a flip chip and a substrate due to the capillary phenomenon.
The language xe2x80x9ccomprising an organic materialxe2x80x9d as used herein is intended to mean that the underfill material does not contain an inorganic filler except pigments.
The present invention embraces the following preferred embodiments:
1) The underfill material further comprises carbon black.
2) The organic material is a one-pack type epoxy resin composition which comprises a low-viscosity epoxy resin and an acid anhydride hardener that is an imide oligomer having an imide unit and an acid anhydride end group, the imide unit being represented by formula (I): 
xe2x80x83wherein A represents an asymmetric aromatic or alicyclic tetracarboxylic acid dianhydride residue; and B represents a diaminopolysiloxane residue,
xe2x80x83and which has a viscosity of 250 P (poise) or less at room temperature (25xc2x0 C.).
3) The one-pack type epoxy resin composition comprises, as an acid anhydride hardener, an imide oligomer having an acid anhydride end group which is obtained by using an alicyclic tetracarboxylic acid dianhydride represented by formula (II): 
xe2x80x83as the asymmetric aromatic or alicyclic tetracarboxylic acid dianhydride.
4) The one-pack type epoxy resin composition comprises an alicyclic acid anhydride hardener and/or a phenolic hardener, and the ratio of the total content of functional groups of all hardeners to the total epoxy group content is one equivalent or less.
5) The one-pack type epoxy resin composition further comprises a silane coupling agent.